The field of the invention generally relates to semiconductor devices, and more particularly relates to thin film resistors and a method of fabricating thin film resistors.
As is well known, integrated circuits and thin film devices frequently require resistors as part of the circuitry, and thin film resistors are commonly used. Thin film resistors generally consist of a thin film of resistive material deposited such as by sputter deposition on a layer or substrate of insulative material with end contacts on the resistive material. The end contacts or interconnections are then connected to circuit components in conventional manner.
As is well known, thin film resistors desirably have a number of characteristics. For example, it is generally desirable that a thin film resistor have at least a minimum thickness in order to handle specified current densities during operation; the thickness should also be relatively uniform. It is also desirable that a thin film resistor have a target or intended resistance commonly expressed as sheet resistance in ohms per square. Further, it is normally desirable that a thin film resistor have a very low temperature coefficient of resistance, or at least a temperature coefficient of resistance that is suitably matched to a particular application. For example, it may be desirable to have a temperature coefficient of resistance that is either positive or negative. Temperature coefficient of resistance is a measure of how the sheet resistance varies with temperature. Therefore, a thin film resistor with a zero coefficient of resistance theoretically does not vary in resistance as the temperature changes. As an example, it may be desirable to fabricate thin film resistors having a sheet resistance in the range from 1000 to 10,000 ohms per square with a temperature coefficient of resistance less than 50 parts per million.
Some materials such as chromium silicon monoxide can be deposited such as by sputter deposition to form high resistance films that have relatively low temperature coefficients of resistance. However, the sheet resistances may generally be unreproducible. For example, integrated circuits are typically thermally processed such as for annealing. As is known, raising the temperature of a thin film resistor normally forms a crystalline structure and the sheet resistance decreases. Therefore, it may be necessary to provide a higher than desired sheet resistance knowing that it will decrease during subsequent thermal processing. However, with certain materials such as chromium silicon monoxide, the sheet resistance may fall dramatically and unpredictably during thermal processing before the integrated circuit is packaged. Therefore, without accurately knowing how much the sheet resistance will fall, it may not be possible to set a sheet resistance during the deposition stage of fabrication so as to arrive at a target or intended sheet resistance later during the fabrication of the integrated circuit. In any event, the yield of such thin film resistors having a specified or target sheet resistance may be very low.
Further, the uniformity of a thin film resistor made of a material such as CrSiO may typically be in the range from 5-15%, and may not be repeatable from run to run. This factor also contributes to the unpredictability of sheet resistance.